This invention relates to an opto-electronic memory system comprising a memory element in which data is stored in rows of optically readable memory locations, electro-optical reading means for reading the rows of memory locations and comprising an array of light photosensitive elements.
Examples of such systems are described in GB-A-2050663, GB-A-2161632 and EP-A-0509597. In these examples, information comprising data is stored in the memory elements in the form of optically differentiatable data bits of differing optical reflectivity or transmissivity. In GB-A-2050663 a perforated card type of data storage element is used whose information is read by feeding the card between aligned linear sets of LEDs and phototransistors. As the card is moved between the arrays, rows of data are read out with the passage of light from an individual LED to its associated phototransistor being determined by the presence or absence of an aperture in the card. In GB-A-2161632 data is stored in an optical memory card as regions of different reflectivity by means of pits, spots or marks. To read this data, a laser beam is scanned over the card using an electro-mechanical drive with retro-reflected laser light being detected by a linear CCD array. In EP-A-0509597, data is stored in memory locations of an optical memory card as regions having different optical transmissivity formed, for example, as transparent and opaque regions in a photographic film. Data read-out is achieved using an electro-optic (liquid crystal) shutter arrangement and a two dimensional array of photosensitive elements disposed on opposite sides of the card. The shutter arrangement illuminates a row of memory locations at a time and the contents of the row are read by the photosensitive elements which are each linear and extend parallel to one another perpendicularly of the row direction.
In these systems optically encoded data can be stored in the memory elements as bits in binary form with each bit location having two optically different states, either transmissive or opaque, or reflective or non-reflective, to represent digital logic values "1" and "0" respectively, or vice versa. Thus each location defines one bit of data.
The data density of the memory element in these systems is limited by the physical size of the photosensitive elements, at least in one dimension, of the light sensing component used to read the memory element for both linear and two-dimensional arrays of photosensitive elements. The resolution of the light sensing component in the reading means is determined by the pitch of the photosensitive elements, and to an extent also by the cone of acceptance of light of those elements. By reducing the pitch, and the cone of acceptance, a greater resolution can be obtained which, in turn, allows the density of the memory locations, and thus amount of data stored, in a memory element of a given size to be increased. However, arrays of photosensitive elements having a smaller pitch are more difficult to fabricate and lower yields can be expected. Consequently, manufacturing costs are increased. Moreover, there are limitations as to how far the photosensitive elements can be reduced in size bearing in mind restrictions to the technology used to fabricate the elements and the ability of the elements to have the required operational characteristics, with the appropriate discrimination, when reading data.
It is mentioned in EP-A-0509597 that it may be possible to record information in the memory element as a multi-level, quasi-analogue, optical transmission factor in the manner of a grey scale with individual memory locations providing one of a series of light attenuation levels. In this way, more than one bit could in effect be stored at each memory location. However, as is pointed out in that specification, the light sensitive elements would then need to be appropriately responsive to different levels of illumination to produce respectively different output signals clearly in order to distinguish the information. In practice, such operation would be difficult to achieve reliably. For smaller photosensitive elements it becomes increasingly difficult to resolve the grey-levels.